Stock ECU is running too lean?

[1969 Datsun 510]

I'm running the stock ECU on my OBDI Nissan SR20DET (s13 red top) motor and everything has been fine for about 15,000 miles. I've always used the same Wideband (PLX) sensor, sending a narrow-band signal to the ECU. Lately under slight load the A/F starts creeping lean. At around 50-80% throttle the A/F will creep up to 16, maybe even 17! I've even seen 20 before on a really hot day because the symptom seems more pronounced with higher ambient temps. However, at higher RPM's at WOT, everything corrects with A/F dropping into the 12's and 11's. What could be causing the lean problem?

Should I replace the 02 sensor? I'd like to run a 2nd sensor to test mine somehow. I've tried another MAFS but had the same symptoms. The fact that it operates properly at WOT elilminates a lot of things like clogged fuel components, bad FPR, etc. Any thoughts?

[1969 Datsun 510]

I noticed this morning that the lean condition seems to go away as soon as I reach WOT. I guess this is when the ECU switches from closed loop to open loop? So in closed loop the A/F is very lean, but once in open loop it is fine. Seems like a o2 sensor problem maybe? How long should a WB o2 sensor last?

[N2UAD]

In general...
The ECU will go into open loop mode with WOT and ignore the O2 sensor. If not too expensive I'd try replacing the O2 sensor.
You can also check the other inputs to the ECU like air temp sensor, engine temp sensor, etc.

[kamesama980]

how many miles on the engine/gaskets? if it's going through any oil, it could be damaging the sensor but that it's reading rich at WOT makes me wonder. maybe try calibrating it.

[1969 Datsun 510]

Not using any oil. It's lean at WOT, not rich. How would one calibrate a WB o2 sensor? Send it back to PLX?

[bobc455]

Please refresh me. Do you have a NB connected to the ECM and a separate WB O2 sensor?

-BC

[1969 Datsun 510]

No, I'm using a PLX (brand) SM-AFR (model) wideband controller and a Bosch 17014 5-wire WB O2 sensor. The controller has an analog output scaled to simulate the output of a narrow-band sensor. This output is connected to the stock ECU.